1. Field of the Invention
The invention relates generally to well logging using a resistivity tool; more particularly, it relates to resistivity logging while drilling in a well drilled with a nonconductive mud.
2. Background Art
In general terms, in order to explore hydrocarbon deposits, it is highly desirable to obtain accurate knowledge of the characteristics of the geological formation at various depths of the borehole. Many of these characteristics are very fine in structure, e.g. stratifications, non-uniform elements, pore characteristics, breaks, etc. By way of example, the orientations, the density, and the lengths of breaks play a major role in the dynamic characteristics of a reservoir rock.
For many years, it has been possible to determine such fine characteristics only by analyzing drill cores taken when drilling the borehole. However the taking of such cores is a technique that is extremely expensive, and use thereof remains relatively exceptional.
Patent EP-0 110 750, or corresponding U.S. Pat. No. 4,567,759, issued to Ekstrom et al., describes a technique of producing an image of the wall of a borehole which consists in generating characteristic signals at regular time intervals representative of a measurement with high spatial resolution of some characteristic of the wall, measuring the depth of the hole to which the characteristic signals relate with accuracy of the same order as the spatial resolution of the characteristic signals, and converting the signals representing the characteristic as a linear function of borehole depth with a color scale being associated with the values of the converted signals in order to form a visual image.
That imaging technique is implemented more particularly with a tool for investigating the conductivity of the formation, as described for example in patent EP-0 071 540, or its corresponding U.S. Pat. No. 4,468,623, issued to Gianzero et al., that is capable of detecting characteristics with millimeter resolution. That type of tool has a series of control electrodes, also known as “buttons”, placed on a conductive pad pressed against the wall of the borehole. A constant current source applies voltage to each button and the conductive surface of the pad so that measurement currents are injected into the formation perpendicularly to the wall. A return is provided for the current by means of an electrode situated close to the surface, or possibly on another part of the tool. The pad is moved along the borehole and the discrete currents associated with each button are proportional to the conductivity of the material facing the buttons.
In application of the teaching of patent U.S. Pat. No. 4,567,759, issued to Ekstrom et al., the signals are modified by eliminating effects such as variations in the speed of the tool and disturbances due to variations in the environment of the tool as amplified and displayed in a manner which comes close to providing a visual image of the inside of the hole. p That imaging technique has been highly successful over the last few years when used in boreholes drilled with conductive drilling mud such as water-base mud or mud of the oil-in-water emulsion type. However, with muds having a continuous non-conductive phase, such as oil-base muds or water-in-oil emulsion type muds, the images obtained are of very poor quality. These poor results are generally attributed to interference due to the presence of a layer of non-conductive mud, or of a layer of mud and a mud cake, interposed between the buttons and the formation under test. Since the thickness of the layer of mud varies in particular as a function of the roughness of the wall, the variations in the resulting currents can completely mask any current variations due to the formation being measured.
Other techniques address measurements in non-conductive mud, among which patent U.S. Pat. No. 6,191,588 discloses a tool for investigating formation conductivity uses a non-conductive pad and buttons that form voltage electrodes instead of current electrodes as described in U.S. Pat. No. 4,468,623. The current injection electrodes are situated off the pad, or in a preferred variant, directly at the ends thereof. In any event, the two injectors are placed in such a manner that current passes through the formation substantially parallel to the pad and thus preferably flows substantially orthogonally to the boundaries of the strata. Under such conditions, the potential difference between two buttons is proportional to the resistivity of the material facing the buttons.
The above-specified U.S. Pat. No. 6,191,588 recommends using DC, or AC at very low frequency, such that the resistivity of the pad is much grater than the resistivity of the drilling mud. However, in practice, working with DC gives rise to problems of noise due in particular to the formation of spontaneous potentials in the formation. In addition, the resistance of the mud limits the quantity of current injected; the potential differences measured between two pairs of buttons are thus very small and therefore difficult to measure.
It would therefore be desirable to be able to work with AC at a relatively high frequency, e.g. on the order of a few thousand hertz. Unfortunately, at such frequencies, the pad behaves like a dielectric whose effective conductivity is similar to that of the mud. This gives rise to an electrical impedance through the pad that is of the same order as the impedance through the layer of mud. Under such conditions, the potential differences between pairs of buttons are more representative of the potential difference applied between the current electrodes than they are of the resistivity of the formation facing them, therefore, the tool becomes unusable.
PCT Patent Application No. WO 01/77710 describes an improvement of the tool disclosed in U.S. Pat. No. 6,191,588 to make it possible to work with AC at frequencies higher than 1000 Hz. Therefore, this patent application provides a tool for investigating the wall of a borehole in a geological formation that comprises a non-conductive pad near the end of which are mounted an AC source electrode and a current return electrode and in the center of which is an array of pairs of voltage difference measurement electrode (dV). The resistivity of the formation opposite each pair of dV electrodes is calculated using:ρ=k. dV/I where ρ is the resistivity, k is a geometrical factor, dV is the voltage difference between a pair of electrodes and I is the current in the formation.
To shield the dV electrodes from the electric field generated in the insulating pad and in the non-conducting fluid, a conducting backplate is included behind the insulating pad, parallel to the front face and covering most of the region between the current electrodes. In a particularly preferred variant of the invention, the electrically conductive portion of the pad is connected to ground, or more precisely it is placed at the same electrical potential as the geological formation. Under such conditions, the measurement electrodes do indeed measure the potential of the formation facing them even when the pad is inclined, i.e. when the “standoff” distance between the formation and the source electrodes is different from the standoff distance between the formation and the return electrode.
The major limitation of the measurement is that the pad must be close to the borehole wall, especially in low-resistivity formations. Otherwise, the dV measurement is sensitive to the electric field generated in the borehole fluid and pad rather than in the formation. For example, in a 0.1 Ω.m formation the maximum standoff is about 5 mm, while in a 100-Ω.m formation the maximum standoff is about 15 mm. As a result, when the borehole is rough the images are spoiled by incorrect readings and become uninterpretable.
To overcome this problem, one approach proposes improved methods of shielding the voltage measurement electrodes from the electric field generated in the pad by the current injectors. The shielding is flush with or almost flush with the outside face of the pad.
Due to these particular features, the apparatus according to the above approach allows for accurate resistivity measurements in non-conductive mud wells, even when the pad is not closely pressed against the formation wall, due to thick mudcake or rugosity of said wall. Due to the shielding means, the electrical field in the pad is eliminated or almost eliminated. In the mud between the pad and the borehole wall, the electrical field is also drastically reduced in the vicinity of the measurement electrodes such that electric equipotential curves in the mud remain almost perpendicular to the formation wall. Therefore, the potential at these measurement electrodes remains close to that in the formation.
In one embodiment, the pad itself constitutes the shielding means, said pad being made of electrically conductive material. In this case, electrically insulating inserts are arranged in the pad around each of the source, return and measurement electrodes.
In a second embodiment, the pad is made of electrically non conductive material and the shielding means comprise electrically conductive sheets that are arranged inside said pad such that said conductive sheets are almost flush with the outside face of said pad.
While the above-described tools are capable of providing images of boreholes drilled with non-conductive muds, they are wireline tools and are not suitable for logging-while-drilling applications. Therefore, a need exists for tools or methods for imaging a borehole while a borehole is being drilled with a non-conductive mud.